fix DDC explicit field initialization to null

This was unintentionally regressed during a fix to field init order. It
does not typically show up in Dart code because undefined and null are
both handled, but it can break JS interop.

Change-Id: I771895826ee09bb6c5e2f9b016eb7673caf61cd1
Reviewed-on: https://dart-review.googlesource.com/40202
Reviewed-by: Vijay Menon <vsm@google.com>

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

@@ -2476,14 +2476,12 @@ class CodeGenerator extends Object
     for (var field in fieldDecls) {
       var f = field.element;
       if (f.isStatic) continue;
-      var init = field.initializer;
-      if (init == null ||
-          ctorFields != null &&
-              ctorFields.contains(f) &&
-              _constants.isFieldInitConstant(field)) {
+      if (ctorFields != null &&
+          ctorFields.contains(f) &&
+          _constants.isFieldInitConstant(field)) {
         continue;
       }
-      emitFieldInit(f, init, field.name);
+      emitFieldInit(f, field.initializer, field.name);
     }
 
     if (ctor != null) {

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

@@ -1470,10 +1470,9 @@ class ProgramCompiler
     for (var f in fields) {
       if (f.isStatic) continue;
       var init = f.initializer;
-      if (init == null ||
-          ctorFields != null &&
-              ctorFields.contains(f) &&
-              _constants.isConstant(init)) {
+      if (ctorFields != null &&
+          ctorFields.contains(f) &&
+          (init == null || _constants.isConstant(init))) {
         continue;
       }
       emitFieldInit(f, init);

tests/language_2/field_test.dart

@@ -4,13 +4,10 @@
 // Dart test program for testing setting/getting of instance fields.
 
 import "package:expect/expect.dart";
-import "package:meta/meta.dart" show virtual;
 
 class First {
   First() {}
-  @virtual
   var a;
-  @virtual
   var b;
 
   addFields() {
@@ -25,8 +22,6 @@ class First {
 }
 
 class Second extends First {
-  // TODO: consider removing once http://b/4254120 is fixed.
-  Second() : super() {}
   var c;
   get a {
     return -12;
@@ -37,6 +32,21 @@ class Second extends First {
   }
 }
 
+class FieldInitializedToNull {
+  int x, y;
+
+  static void test() {
+    var f = new FieldInitializedToNull();
+    int missingArg([int x = 42]) => x;
+    Expect.isNull(f.x);
+    Expect.isNull(f.y);
+    // Regression tests for a DDC bug, where undefined gets initialized in the
+    // fields, and is incorrect recognized as a missing argument.
+    Expect.isNull(missingArg(f.x));
+    Expect.isNull(missingArg(f.y));
+  }
+}
+
 class FieldTest {
   static one() {
     var f = new First();
@@ -68,13 +78,10 @@ class FieldTest {
     o.b = o;
     Expect.equals(12, o.c);
   }
-
-  static testMain() {
-    // FieldTest.one();
-    FieldTest.two();
-  }
 }
 
 main() {
-  FieldTest.testMain();
+  FieldTest.one();
+  FieldTest.two();
+  FieldInitializedToNull.test();
 }

tests/lib_2/html/js_typed_interop_test.dart

@@ -78,6 +78,8 @@ _injectJs() {
 
   function confuse(obj) { return obj; }
 
+  function isUndefined(obj) { return obj === void 0; }
+
   function StringWrapper(str) {
     this.str = str;
   }
@@ -191,6 +193,9 @@ class StringWrapper {
 @JS()
 external confuse(obj);
 
+@JS()
+external isUndefined(obj);
+
 @JS()
 external CanvasRenderingContext2D getCanvasContext();
 
@@ -200,6 +205,10 @@ external num get propertyOnDocument;
 @JS('window.self.window.window.windowProperty')
 external num get propertyOnWindow;
 
+class DartClassWithNullField {
+  int x;
+}
+
 main() {
   _injectJs();
 
@@ -433,6 +442,7 @@ main() {
       expect(selection is List, isTrue);
     });
   });
+
   group('html', () {
     test('return html type', () {
       expect(getCanvasContext() is CanvasRenderingContext2D, isTrue);
@@ -442,4 +452,8 @@ main() {
       expect(propertyOnDocument, equals(45));
     });
   });
+
+  test('Dart field is null instead of undefined', () {
+    expect(isUndefined(new DartClassWithNullField().x), false);
+  });
 }