Don't report redundant type errors in strong mode.

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 54669495a51c45d67f08d22889cf2afce61b083e..715ad1cc0c38d1109df9090973e514de116eac77 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -151,18 +151,19 @@ abstract class Iterable<E> {
       /*=R*/ combine(/*=R*/ previousValue, E element));
 }
 
-abstract class List<E> implements Iterable<E> {
-  void add(E value);
-  E operator [](int index);
-  void operator []=(int index, E value);
+class List<E> implements Iterable<E> {
+  List();

[Brian Wilkerson, 2016/03/10 00:09:33]

What's the motivation for these changes? Both List and Map are abstract in the
SDK, so it seems like they ought to be that way in the mock SDK.

[Bob Nystrom, 2016/03/10 00:40:55]

strong_mode_test uses `new List()` in a few tests. Before this CL, that was fine
since the strong mode tests didn't validate any non-strong mode errors at all.
Now that they do, it was producing spurious "can't construct an abstract class"
warnings. I figured the simple solution was to add the constructor here, but I
can do something different if you'd like.

[Brian Wilkerson, 2016/03/10 00:44:22]

I believe that we've been reproducing the SDK exactly except that it's a subset.
That would require adding a factory constructor, and a fake private
implementation. Not sure which is better. I'll leave it to your discretion.

+  void add(E value) {}
+  E operator [](int index) => null;
+  void operator []=(int index, E value) {}
   Iterator<E> get iterator => null;
-  void clear();
+  void clear() {}
 }
 
-abstract class Map<K, V> extends Object {
-  Iterable<K> get keys;
-  V operator [](K key);
-  void operator []=(K key, V value);
+class Map<K, V> extends Object {
+  Iterable<K> get keys => null;
+  V operator [](K key) => null;
+  void operator []=(K key, V value) {}
 }
 
 external bool identical(Object a, Object b);