Only generate assert in checked mode.

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
===================================================================
--- lib/compiler/implementation/compiler.dart	(revision 8424)
+++ lib/compiler/implementation/compiler.dart	(working copy)
@@ -79,6 +79,7 @@
   Namer namer;
   Types types;
   final bool enableTypeAssertions;
+  final bool enableUserAssertions;
 
   final Tracer tracer;
 
@@ -102,6 +103,7 @@
   ClassElement functionClass;
   ClassElement nullClass;
   ClassElement listClass;
+  Element assertMethod;
 
   Element get currentElement() => _currentElement;
   withCurrentElement(Element element, f()) {
@@ -143,6 +145,7 @@
 
   Compiler([this.tracer = const Tracer(),
             this.enableTypeAssertions = false,
+            this.enableUserAssertions = false,
             bool emitJavascript = true,
             validateUnparse = false])
       : libraries = new Map<String, LibraryElement>(),
@@ -323,6 +326,8 @@
     libraries['dart:core'] = coreLibrary;
     libraries['dart:coreimpl'] = coreImplLibrary;
 
+    assertMethod = coreLibrary.find(const SourceString('assert'));
+
     initializeSpecialClasses();
   }