Implement messages in assert() in dev_compiler.

pkg/dev_compiler/lib/js/es6/dart_sdk.js

Index: pkg/dev_compiler/lib/js/es6/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/es6/dart_sdk.js b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
index 18df1635520c40e0785b9a992c4a902ea3d0151d..f89c44472c5879438c7df13a22e1a26b0cc63826 100644
--- a/pkg/dev_compiler/lib/js/es6/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
@@ -1534,6 +1534,10 @@ dart.throwAssertionError = function() {
   debugger;
   dart.throw(new core.AssertionError());
 };
+dart.throwAssertionErrorWithMessage = function(message) {
+  debugger;
+  dart.throw(new _js_helper.AssertionErrorWithMessage(message));
+};
 dart.throwNullValueError = function() {
   debugger;
   dart.throw(new core.NoSuchMethodError(null, new core.Symbol('<Unexpected Null Value>'), null, null, null));
@@ -1896,6 +1900,9 @@ dart.map = function(values, K, V) {
 dart.assert = function(condition) {
   if (!condition) dart.throwAssertionError();
 };
+dart.assertMessage = function(condition, message) {
+  if (!condition) dart.throwAssertionErrorWithMessage(message());
+};
 dart.throw = function(obj) {
   dart._stack = new Error();
   throw obj;
@@ -11877,6 +11884,30 @@ dart.setSignature(_js_helper.RuntimeError, {
   constructors: () => ({new: dart.definiteFunctionType(_js_helper.RuntimeError, [dart.dynamic])}),
   fields: () => ({message: dart.dynamic})
 });
+core.AssertionError = class AssertionError extends core.Error {
+  new() {
+    super.new();
+  }
+  toString() {
+    return "Assertion failed";
+  }
+};
+dart.setSignature(core.AssertionError, {
+  constructors: () => ({new: dart.definiteFunctionType(core.AssertionError, [])})
+});
+_js_helper.AssertionErrorWithMessage = class AssertionErrorWithMessage extends core.AssertionError {
+  new(message) {
+    this[_message] = message;
+    super.new();
+  }
+  toString() {
+    return dart.str`Assertion failed: ${this[_message]}`;
+  }
+};
+dart.setSignature(_js_helper.AssertionErrorWithMessage, {
+  constructors: () => ({new: dart.definiteFunctionType(_js_helper.AssertionErrorWithMessage, [core.Object])}),
+  fields: () => ({[_message]: core.Object})
+});
 _js_helper.random64 = function() {
   let int32a = Math.random() * 0x100000000 >>> 0;
   let int32b = Math.random() * 0x100000000 >>> 0;
@@ -12007,17 +12038,6 @@ _js_helper.SyncIterable$ = dart.generic(E => {
   return SyncIterable;
 });
 _js_helper.SyncIterable = SyncIterable();
-core.AssertionError = class AssertionError extends core.Error {
-  new() {
-    super.new();
-  }
-  toString() {
-    return "Assertion failed";
-  }
-};
-dart.setSignature(core.AssertionError, {
-  constructors: () => ({new: dart.definiteFunctionType(core.AssertionError, [])})
-});
 _js_helper.BooleanConversionAssertionError = class BooleanConversionAssertionError extends core.AssertionError {
   new() {
     super.new();