Implement type cast in the VM.

runtime/lib/error.dart

Index: runtime/lib/error.dart
===================================================================
--- runtime/lib/error.dart	(revision 9048)
+++ runtime/lib/error.dart	(working copy)
@@ -48,6 +48,25 @@
   final String malformedError;
 }
 
+class CastException extends TypeError {
+  factory CastException._uninstantiable() {
+    throw const UnsupportedOperationException(
+        "CastException can only be allocated by the VM");
+  }
+  // A CastException is allocated by TypeError._throwNew() with "type cast" as
+  // dst_name.
+  String toString() {
+    String str = (malformedError != null) ? malformedError : "";
+    if ((dstName != null) && (dstName.length > 0)) {
+      str = "${str}type '$srcType' is not a subtype of "
+            "type '$dstType' in type cast.";
+    } else {
+      str = "${str}malformed type used in type cast.";
+    }
+    return str;
+  }
+}
+
 class FallThroughError {
   factory FallThroughError._uninstantiable() {
     throw const UnsupportedOperationException(