Throw TypeError instead of CastError for type coercions.

tool/input_sdk/private/js_helper.dart

Index: tool/input_sdk/private/js_helper.dart
diff --git a/tool/input_sdk/private/js_helper.dart b/tool/input_sdk/private/js_helper.dart
index 9048d0facc5d6e72471edf702ef79b1e60849278..8df5529ac0209e2256bd1d3e9f4bd67cd3acf5dd 100644
--- a/tool/input_sdk/private/js_helper.dart
+++ b/tool/input_sdk/private/js_helper.dart
@@ -782,9 +782,9 @@ class TypeErrorImplementation extends Error implements TypeError {
   /**
    * Normal type error caused by a failed subtype test.
    */
-  TypeErrorImplementation(Object value, String type)
-      : message = "type '${Primitives.objectTypeName(value)}' is not a subtype "
-                  "of type '$type'";
+  TypeErrorImplementation(Object value, Object actualType, Object expectedType)
+      : message = "Type '${actualType}' is not a subtype "
+                  "of type '${expectedType}'";
 
   TypeErrorImplementation.fromMessage(String this.message);
 
@@ -799,13 +799,40 @@ class CastErrorImplementation extends Error implements CastError {
   /**
    * Normal cast error caused by a failed type cast.
    */
-  CastErrorImplementation(Object actualType, Object expectedType)
-      : message = "CastError: Casting value of type $actualType to"
-                  " incompatible type $expectedType";
+  CastErrorImplementation(Object value, Object actualType, Object expectedType)
+      : message = "CastError: Casting value of type '$actualType' to"
+                  " incompatible type '$expectedType'";
 
   String toString() => message;
 }
 
+/// Thrown by type assertions that fail in strong mode that would have passed in
+/// standard Dart.
+class StrongModeTypeError extends Error implements TypeError, StrongModeError {
+  final String message;
+  StrongModeTypeError(Object value, Object actualType, Object expectedType)
+      : message = "Type '${actualType}' is not a subtype "
+                  "of type '${expectedType}' in strong mode";
+  String toString() => message;
+}
+
+/// Thrown by casts that fail in strong mode that would have passed in standard
+/// Dart.
+class StrongModeCastError extends Error implements CastError, StrongModeError {
+  final String message;
+  StrongModeCastError(Object value, Object actualType, Object expectedType)
+      : message = "CastError: Casting value of type '$actualType' to"
+                  " type '$expectedType' which is incompatible in strong mode";
+  String toString() => message;
+}
+
+/// Used for Strong-mode errors other than type assertions and casts.
+class StrongModeErrorImplementation extends Error implements StrongModeError {
+  final String message;
+  StrongModeErrorImplementation(this.message);
+  String toString() => message;
+}
+
 class FallThroughErrorImplementation extends FallThroughError {
   FallThroughErrorImplementation();
   String toString() => "Switch case fall-through.";