fix(BaseException): convert JS stack property to getter

The stack property of BaseException was previously being set in
the constructor based on the provided message, but subclasses 
of BaseException would change the message after constructing the 
error. This change implements stack as a getter of BaseException,
deferring the creation of the stack until it is asked for.

Fixes angular#2570

modules/angular2/src/facade/lang.dart

@@ -175,7 +175,7 @@ class FunctionWrapper {
 }
 
 class BaseException extends Error {
-  final String message;
+  String message;
 
   BaseException([this.message]);
 

modules/angular2/src/facade/lang.ts

@@ -7,14 +7,14 @@ export type Type = new (...args: any[]) => any;
 
 export class BaseException extends Error {
   message;
-  stack;
   constructor(message?: string) {
     super(message);
     this.message = message;
-    this.stack = (<any>new Error(message)).stack;
   }
 
   toString(): string { return this.message; }
+
+  get stack() { return (<any>new Error(this.message)).stack }
 }
 
 export var Math = _global.Math;

modules/angular2/test/di/exceptions_spec.dart

@@ -0,0 +1,3 @@
+library angular2.test.di.exceptions_spec;
+// Not implemented
+main() {}

modules/angular2/test/di/exceptions_spec.ts

@@ -0,0 +1,24 @@
+import {
+  AsyncTestCompleter,
+  beforeEach,
+  ddescribe,
+  describe,
+  expect,
+  iit,
+  inject,
+  it,
+  xit,
+} from 'angular2/test_lib';
+import {AbstractBindingError} from 'angular2/di';
+
+export function main() {
+  describe('exceptions', () => {
+    it('should have the proper message when thrown', () => {
+      try {
+        throw new AbstractBindingError('foo', () => { return 'message'; });
+      } catch (e) {
+        expect(e.stack.split('\n')[0]).toBe('Error: message');
+      }
+    });
+  });
+}

modules/angular2/test/facade/lang_spec.ts

@@ -6,10 +6,30 @@ import {
   RegExpWrapper,
   RegExpMatcherWrapper,
   StringWrapper,
-  CONST_EXPR
+  CONST_EXPR,
+  BaseException,
+  isString
 } from 'angular2/src/facade/lang';
 
 export function main() {
+  describe('BaseException', () => {
+    it('should include error message in stack, if added after initialization', () => {
+      try {
+        var exception = new BaseException();
+        exception.message = 'Msg';
+        throw exception;
+      } catch (e) {
+        if (isString(e.stack)) {
+          // In JS, the stack contains the message, so we verify that added message is in the stack
+          expect(`${e.stack}`.split('\n')[0]).toBe('Error: Msg');
+        } else {
+          // In Dart, the stack does not contain the message, so we toString the error directly
+          expect(e.toString()).toBe('Msg');
+        }
+      }
+    });
+  });
+
   describe('RegExp', () => {
     it('should expose the index for each match', () => {
       var re = RegExpWrapper.create('(!)');