Implement 'as' operator.

lib/compiler/implementation/lib/mock.dart

Index: lib/compiler/implementation/lib/mock.dart
diff --git a/lib/compiler/implementation/lib/mock.dart b/lib/compiler/implementation/lib/mock.dart
index 89e26be7fe34929bedd1de454fa3c135ea42d88c..d5e601c6756d9f6fa88b4a63743e29dcbbf5f3ba 100644
--- a/lib/compiler/implementation/lib/mock.dart
+++ b/lib/compiler/implementation/lib/mock.dart
@@ -13,6 +13,21 @@ class TypeError extends AssertionError {
   String toString() => msg;
 }
 
+/** Thrown by the 'as' operator if the cast isn't valid. */
+class CastException implements TypeError {
+  // TODO(lrn): Change actualType and expectedType to "Type" when reified
+  // types are available.
+  final Object actualType;
+  final Object expectedType;
+
+  CastException(this.actualType, this.expectedType);
+
+  String toString() {
+    return "CastException: Casting value of type $actualType to"
+           " incompatible type $expectedType";
+  }
+}
+
 class FallThroughError {
   const FallThroughError();
   String toString() => "Switch case fall-through.";