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 {

[rmacnak, 2016/10/10 21:29:39]

DBC: Don't make this type public.

[sra1, 2016/10/10 21:50:40]

This class also throws away information. You can't inspect it to see the
arguments.

If you change the VM NoSuchMethodError formatting, then you can still inspect
the exception object in the debugger to see the arguments.

All the information needed to do a nicer job of presentation is already
available on NoSuchMethodError. There is already a lot of logic in there to help
users. Improve it.

You could change the print logic for a null receiver, for getters, or both. e.g.
from

NoSuchMethodError: method not found: 'isDeferredLoaderGetter'
Receiver: null
Arguments: []

to something like one of the following

NoSuchMethodError: null does not have a getter called 'isDeferredLoaderGetter'

NullError: getter not found: 'null.isDeferredLoaderGetter'

[sra1, 2016/10/10 21:50:40]

Yes, please remove from corelib.
We can't reliably support

    e is NullDereferenceError

in dart2js.

+  /** Message describing the problem. */
+  final message;
+
+  /**
+   * The [message] optionally describes the circumstances
+   * under which [:null:] was dereferenced.
+   */
+  NullDereferenceError([this.message]);
+
+  String toString() {
+    return "Cannot dereference null.${ message != null ? ' $message' : ''}";

[rmacnak, 2016/10/10 21:29:39]

DBC: There is no such operation as "dereferencing" in Dart, only method
invocation. Also, `null.toString()`, `null.runtimeType`, etc do not cause any
exception. Consider something more along the lines of "unexpected invocation on
null".

[sra1, 2016/10/10 21:50:40]

Still use the 'no such method' terminology.

This message adds confusion because you can 'dereference' null, e.g.

null.toString()   --> "null"
null.toString(1)  --> throws NoSuchMethodError

+  }
+}
+
+/**
  * 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. */
@@ skipping 431 matching lines @@
  * 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";
 }