Give useful message for null derefs

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 c11ea2e1a97da25e0c73490a216d7ec025492dc6..6dce2af78dcaf0286d4585d327b33144656a807a 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -120,6 +120,21 @@ class NullThrownError extends Error {
 }
 
 /**
+ * Error thrown when attempting to dereference [:null:].
+ */
+class NullDereferenceError extends Error implements NoSuchMethodError {
+  /** Message describing the problem. */
+  final message;
+
+  /**
+   * The [message] optionally describes the circumstances under which [:null:] was dereferenced.

[Cutch, 2016/10/07 23:24:11]

80 columns...

+   */
+  NullDereferenceError([this.message]);
+
+  String toString() => "Cannot dereference null.${ message != null ? ' $message' : ''}";
+}
+
+/**
  * Error thrown when a function is passed an unacceptable argument.
  */
 class ArgumentError extends Error {