Give useful message for null derefs

sdk/lib/core/errors.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * Error objects thrown in the case of a program failure.
  *
  * An `Error` object represents a program failure that the programmer
@@ skipping 102 matching lines @@
 }
 
 /**
  * Error thrown when attempting to throw [:null:].
  */
 class NullThrownError extends Error {
   String toString() => "Throw of null.";
 }
 
 /**
+ * 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 ? ' $message' : ''}";
+}
+
+/**
  * Error thrown when a function is passed an unacceptable argument.
  */
 class ArgumentError extends Error {
   /** Whether value was provided. */
   final bool _hasValue;
   /** The invalid value. */
   final invalidValue;
   /** Name of the invalid argument, if available. */
   final String name;
   /** Message describing the problem. */
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  * the first time it is read. If evaluating the initializer expression causes
  * another read of the variable, this error is thrown.
  */
 class CyclicInitializationError extends Error {
   final String variableName;
   CyclicInitializationError([this.variableName]);
   String toString() => variableName == null
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }