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..8e7710889f9fce774d92a209b054f00e50033325 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -120,6 +120,24 @@ class NullThrownError extends Error {
 }
 
 /**
+ * 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 {