Make RuntimeError an Error

sdk/lib/core/errors.dart

Index: sdk/lib/core/errors.dart
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index eeb0c04fdef2e4f25040354dc318709b320b4728..c18e174e0190334025f5a67b5a23fd1127245d58 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -245,3 +245,12 @@ class StackOverflowError implements Error {
   const StackOverflowError();
   String toString() => "Stack Overflow";
 }
+
+/**
+ * Error thrown when a runtime error occurs.
+ */
+class RuntimeError implements Error {
+  final message;
+  RuntimeError(this.message);
+  String toString() => "RuntimeError: $message";
+}