Expose remaining 'errors' in an unified dart:core.

runtime/lib/error.dart

Index: runtime/lib/error.dart
diff --git a/runtime/lib/error.dart b/runtime/lib/error.dart
index 29a071a670d6e6f2c6fe998972cd3fdf4451a809..2305f378920ce19d4f6f1255274f646cfc2ac5cd 100644
--- a/runtime/lib/error.dart
+++ b/runtime/lib/error.dart
@@ -4,7 +4,7 @@
 // Errors are created and thrown by DartVM only.
 // Changes here should also be reflected in corelib/error.dart as well
 
-class AssertionErrorImplementation extends AssertionError {
+class AssertionErrorImplementation implements AssertionError {
   factory AssertionErrorImplementation._uninstantiable() {
     throw const UnsupportedOperationException(
         "AssertionError can only be allocated by the VM");
@@ -21,8 +21,10 @@ class AssertionErrorImplementation extends AssertionError {
   final int column;
 }
 
-class TypeError extends AssertionErrorImplementation {
-  factory TypeError._uninstantiable() {
+class TypeErrorImplementation
+    extends AssertionErrorImplementation
+    implements TypeError {
+  factory TypeErrorImplementation._uninstantiable() {
     throw const UnsupportedOperationException(
         "TypeError can only be allocated by the VM");
   }
@@ -48,7 +50,9 @@ class TypeError extends AssertionErrorImplementation {
   final String malformedError;
 }
 
-class CastException extends TypeError {
+class CastExceptionImplementation
+    extends TypeErrorImplementation
+    implements CastException {
   factory CastException._uninstantiable() {
     throw const UnsupportedOperationException(
         "CastException can only be allocated by the VM");