Implement messages in assert() in dev_compiler.

pkg/dev_compiler/lib/src/compiler/code_generator.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 3801 matching lines @@
 
   @override
   JS.Statement visitExpressionStatement(ExpressionStatement node) =>
       _visit(node.expression).toStatement();
 
   @override
   JS.EmptyStatement visitEmptyStatement(EmptyStatement node) =>
       new JS.EmptyStatement();
 
   @override
-  JS.Statement visitAssertStatement(AssertStatement node) =>
-      // TODO(jmesserly): only emit in checked mode.
-      _callHelperStatement('assert(#);', _visit(node.condition));
+  JS.Statement visitAssertStatement(AssertStatement node) {
+    // TODO(jmesserly): only emit in checked mode.
+    if (node.message != null) {
+      return _callHelperStatement('assertMessage(#, () => #);',

[Jennifer Messerly, 2016/12/12 22:36:01]

i'd prefer this to be an optional parameter to dart.assert, rather than a second
runtime function.

(my thought process for generating readable JS is: what would a JS user and/or
library author do?)

so this would be 'assert(#, () => #)'

[Bob Nystrom, 2016/12/13 19:29:51]

Done. I wanted to distinguish between an explicit null message (which should
print "Assertion failed: null", not "Assertion failed" and no message. That
meant I couldn't do "!= null" later to decide which kind of error to show.

But I realize now that since the message is always thunked, I can check for null
because an explicit null message will show up as a thunk that returns null, not
a bare null.

+          [_visit(node.condition), _visit(node.message)]);
+    }
+
+    return _callHelperStatement('assert(#);', _visit(node.condition));
+  }
 
   @override
   JS.Statement visitReturnStatement(ReturnStatement node) {
     var e = node.expression;
     if (e == null) return new JS.Return();
     return (_visit(e) as JS.Expression).toReturn();
   }
 
   @override
   JS.Statement visitYieldStatement(YieldStatement node) {
@@ skipping 1975 matching lines @@
   if (targetIdentifier.staticElement is! PrefixElement) return false;
   var prefix = targetIdentifier.staticElement as PrefixElement;
 
   // The library the prefix is referring to must come from a deferred import.
   var containingLibrary = resolutionMap
       .elementDeclaredByCompilationUnit(target.root as CompilationUnit)
       .library;
   var imports = containingLibrary.getImportsWithPrefix(prefix);
   return imports.length == 1 && imports[0].isDeferred;
 }