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

corelib/src/errors.dart

Index: corelib/src/errors.dart
diff --git a/corelib/src/errors.dart b/corelib/src/errors.dart
index 265f657914fc52e7eb37da97c4524ca3b01cb67d..54223dd1ce77aae0e06ddb69629b6f2bf8b8f229 100644
--- a/corelib/src/errors.dart
+++ b/corelib/src/errors.dart
@@ -4,3 +4,9 @@
 
 class AssertionError {
 }
+
+class TypeError extends AssertionError {
+}
+
+class CastException extends TypeError {
+}