[google_maps_flutter_web] Migrate to null-safety. (flutter#3726)

* Updates JS-interop package to google_maps ^5.1.0
* Breaking changes:
  * The property `icon` of a `Marker` cannot be `null`. Defaults to `BitmapDescriptor.defaultMarker`
  * The property `initialCameraPosition` of a `GoogleMapController` can't be `null`. It is also marked as `required`.
  * The parameter `creationId` of the `buildView` method cannot be `null` (this should be handled internally for users of the plugin)
  * Most of the Controller methods can't be called after `remove`/`dispose`. Calling these methods now will throw an Assertion error. Before it'd be a no-op, or a null-pointer exception.

Author: ditman

packages/google_maps_flutter/google_maps_flutter_web/CHANGELOG.md

@@ -1,3 +1,12 @@
+## 0.3.0
+
+* Migrate package to null-safety.
+* **Breaking changes:**
+  * The property `icon` of a `Marker` cannot be `null`. Defaults to `BitmapDescriptor.defaultMarker`
+  * The property `initialCameraPosition` of a `GoogleMapController` can't be `null`. It is also marked as `required`.
+  * The parameter `creationId` of the `buildView` method cannot be `null` (this should be handled internally for users of the plugin)
+  * Most of the Controller methods can't be called after `remove`/`dispose`. Calling these methods now will throw an Assertion error. Before it'd be a no-op, or a null-pointer exception.
+
 ## 0.2.1
 
 * Move integration tests to `example`.

packages/google_maps_flutter/google_maps_flutter_web/example/README.md

@@ -19,3 +19,13 @@ Make sure you have updated to the latest Flutter master.
 
     * Single: `./run_test.sh integration_test/TEST_NAME.dart`
     * All: `./run_test.sh`
+
+## Mocks
+
+There's new `.mocks.dart` files next to the test files that use them.
+
+Mock files are [generated by `package:mockito`](https://github.com/dart-lang/mockito/blob/master/NULL_SAFETY_README.md#code-generation). The contents of these files can change with how the mocks are used within the tests, in addition to actual changes in the APIs they're mocking.
+
+Mock files can be updated either manually by running the following command: `flutter pub run build_runner build` (or the `regen_mocks.sh` script), or automatically on each call to the `run_test.sh` script.
+
+Please, add whatever changes show up in mock files to your PRs, or CI will fail.

packages/google_maps_flutter/google_maps_flutter_web/example/build.yaml

@@ -0,0 +1,6 @@
+targets:
+  $default:
+    sources:
+      - integration_test/*.dart
+      - lib/$lib$
+      - $package$