Move /language images to be managed within /assets (#5192)

This simplifies our site setup, as we work towards `/assets` being the
standard location for all asset like files.

Contributes to #5193

Contributes to #4473 by
allowing any related image optimization to be consolidated to one
location.

Contributes to #5177 by
making asset pass-through easier to setup.

src/language/concurrency/index.md → src/language/concurrency.md

@@ -160,7 +160,7 @@ the Dart code pauses while `readAsString()` executes non-Dart code,
 in either the Dart virtual machine (VM) or the operating system (OS).
 Once `readAsString()` returns a value, Dart code execution resumes.
 
-![Flowchart-like figure showing app code executing from start to exit, waiting for native I/O in between](/language/concurrency/images/basics-await.png)
+![Flowchart-like figure showing app code executing from start to exit, waiting for native I/O in between](/assets/img/language/concurrency/basics-await.png)
 
 If you'd like to learn more about using `async`, `await`, and futures,
 visit the [asynchronous programming codelab][].
@@ -209,7 +209,7 @@ Dart programs run in the main isolate by default.
 It's the thread where a program starts to run and execute, 
 as shown in the following figure:
 
-![A figure showing a main isolate, which runs `main()`, responds to events, and then exits](/language/concurrency/images/basics-main-isolate.png)
+![A figure showing a main isolate, which runs `main()`, responds to events, and then exits](/assets/img/language/concurrency/basics-main-isolate.png)
 
 Even single-isolate programs can execute smoothly.
 Before continuing to the next line of code, these apps use
@@ -232,7 +232,7 @@ When the isolate's initial function returns,
 the isolate stays around if it needs to handle events.
 After handling the events, the isolate exits.
 
-![A more general figure showing that any isolate runs some code, optionally responds to events, and then exits](/language/concurrency/images/basics-isolate.png)
+![A more general figure showing that any isolate runs some code, optionally responds to events, and then exits](/assets/img/language/concurrency/basics-isolate.png)
 
 
 ### Event handling
@@ -243,15 +243,15 @@ For example, the following figure shows a repaint event,
 followed by a tap event, followed by two repaint events.
 The event loop takes events from the queue in first in, first out order.
 
-![A figure showing events being fed, one by one, into the event loop](/language/concurrency/images/event-loop.png)
+![A figure showing events being fed, one by one, into the event loop](/assets/img/language/concurrency/event-loop.png)
 
 Event handling happens on the main isolate after `main()` exits.
 In the following figure, after `main()` exits,
 the main isolate handles the first repaint event.
 After that, the main isolate handles the tap event,
 followed by a repaint event.
 
-![A figure showing the main isolate executing event handlers, one by one](/language/concurrency/images/event-handling.png)
+![A figure showing the main isolate executing event handlers, one by one](/assets/img/language/concurrency/event-handling.png)
 
 If a synchronous operation takes too much processing time,
 the app can become unresponsive.
@@ -260,7 +260,7 @@ so subsequent events are handled too late.
 The app might appear to freeze,
 and any animation it performs might be jerky.
 
-![A figure showing a tap handler with a too-long execution time](/language/concurrency/images/event-jank.png)
+![A figure showing a tap handler with a too-long execution time](/assets/img/language/concurrency/event-jank.png)
 
 In client apps, the result of a too-lengthy synchronous operation is often
 [janky (non-smooth) UI animation][jank].
@@ -282,7 +282,7 @@ The worker isolate returns its result in a message when the worker exits.
 
 [json]: {{site.flutter-docs}}/cookbook/networking/background-parsing
 
-![A figure showing a main isolate and a simple worker isolate](/language/concurrency/images/isolate-bg-worker.png)
+![A figure showing a main isolate and a simple worker isolate](/assets/img/language/concurrency/isolate-bg-worker.png)
 
 Each isolate message can deliver one object,
 which includes anything that's transitively reachable from that object.
@@ -406,7 +406,7 @@ Previous example's diagram and text for reference:
   The following figure illustrates the communication between
   the main isolate and the worker isolate:
 
-  ![A figure showing the previous snippets of code running in the main isolate and in the worker isolate](/language/concurrency/images/isolate-api.png)
+  ![A figure showing the previous snippets of code running in the main isolate and in the worker isolate](/assets/img/language/concurrency/isolate-api.png)
 {% endcomment %}
 
 #### Sending closures with isolates
@@ -467,7 +467,7 @@ One common pattern, which the following figure shows,
 is for the main isolate to send a request message to the worker isolate,
 which then sends one or more reply messages.
 
-![A figure showing the main isolate spawning the isolate and then sending a request message, which the worker isolate responds to with a reply message; two request-reply cycles are shown](/language/concurrency/images/isolate-custom-bg-worker.png)
+![A figure showing the main isolate spawning the isolate and then sending a request message, which the worker isolate responds to with a reply message; two request-reply cycles are shown](/assets/img/language/concurrency/isolate-custom-bg-worker.png)
 
 Check out the [long_running_isolate.dart][] sample,
 which shows how to spawn a long-running isolate

src/language/type-system.md

@@ -129,7 +129,7 @@ Here are some of the less obvious rules:
 Let's see these rules in detail, with examples that use the following
 type hierarchy:
 
-<img src="images/type-hierarchy.png" alt="a hierarchy of animals where the supertype is Animal and the subtypes are Alligator, Cat, and HoneyBadger. Cat has the subtypes of Lion and MaineCoon">
+<img src="/assets/img/language/type-hierarchy.png" alt="a hierarchy of animals where the supertype is Animal and the subtypes are Alligator, Cat, and HoneyBadger. Cat has the subtypes of Lion and MaineCoon">
 
 <a name="use-proper-return-types"></a>
 ### Use sound return types when overriding methods
@@ -398,7 +398,7 @@ or a producer.
 
 Consider the following type hierarchy:
 
-<img src="images/type-hierarchy.png" alt="a hierarchy of animals where the supertype is Animal and the subtypes are Alligator, Cat, and HoneyBadger. Cat has the subtypes of Lion and MaineCoon">
+<img src="/assets/img/language/type-hierarchy.png" alt="a hierarchy of animals where the supertype is Animal and the subtypes are Alligator, Cat, and HoneyBadger. Cat has the subtypes of Lion and MaineCoon">
 
 Consider the following simple assignment where `Cat c` is a _consumer_
 and `Cat()` is a _producer_:
@@ -445,7 +445,7 @@ Are the rules the same for generic types? Yes. Consider the hierarchy
 of lists of animals—a `List` of `Cat` is a subtype of a `List` of
 `Animal`, and a supertype of a `List` of `MaineCoon`:
 
-<img src="images/type-hierarchy-generics.png" alt="List<Animal> -> List<Cat> -> List<MaineCoon>">
+<img src="/assets/img/language/type-hierarchy-generics.png" alt="List<Animal> -> List<Cat> -> List<MaineCoon>">
 
 In the following example, 
 you can assign a `MaineCoon` list to `myCats`
@@ -489,7 +489,7 @@ depending on the actual type of the list being cast (`myAnimals`).
 When overriding a method, the producer and consumer rules still apply.
 For example:
 
-<img src="images/consumer-producer-methods.png" alt="Animal class showing the chase method as the consumer and the parent getter as the producer">
+<img src="/assets/img/language/consumer-producer-methods.png" alt="Animal class showing the chase method as the consumer and the parent getter as the producer">
 
 For a consumer (such as the `chase(Animal)` method), you can replace
 the parameter type with a supertype. For a producer (such as