Add support for null-aware operators to the CPS-IR.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index b84a3786c9f7e2776b3731a97ca3d1d63889649b..e6e8a9c5cd1d878d7d5f439002f859e248d46062 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -2023,6 +2023,9 @@ abstract class IrBuilder {
                                  {bool isTypeTest: false,
                                   bool isNotCheck: false});
 
+  /// Creates a type test checking whether [value] is null.
+  ir.Primitive buildChecknull(ir.Primitive value);
+
   /// Create a lazy and/or expression. [leftValue] is the value of the left
   /// operand and [buildRightValue] is called to process the value of the right
   /// operand in the context of its own [IrBuilder].
@@ -2323,6 +2326,12 @@ class DartIrBuilder extends IrBuilder {
             (k) => new ir.TypeOperator(receiver, type, k, isTypeTest: isTypeTest));
     return isNotCheck ? buildNegation(check) : check;
   }
+
+  @override
+  ir.Primitive buildCheckNull(ir.Primitive value) {

[Johnni Winther, 2015/05/29 10:23:24]

Move the implementation to IrBuilder.

[Siggi Cherem (dart-lang), 2015/05/29 18:14:05]

After addressing Kevin's comment, this moved just to the JsIrBuilder - the plain
IrBuilder declares the 2 operations, but the DartIrBuilder throws on both

+    assert(isOpen);
+    return addPrimitive(new ir.Identical(value, buildNullConstant()));

[Kevin Millikin (Google), 2015/05/29 11:02:03]

Identical is a 'JS specific' IR primitive, which means that it probably doesn't
work for dart2dart.  Either something will be unimplemented or we will hit an
assertion failure.

In dart2dart we would want to support .? and ?? in the output anyway, which
would either require supporting them directly in the CPS IR or desugaring them
and then recognizing them later.  Both of those choices are a lot of work.

Instead, just giveup in dart mode by throwing an informative string.

[Siggi Cherem (dart-lang), 2015/05/29 18:14:05]

Great points - I moved most of the implementation to the JsIrBuilder and made
the dart version throw.

[Siggi Cherem (dart-lang), 2015/05/29 19:02:02]

Just synced and noticed that DartIrBuilder is gone, nice!... I've kept the
implementation in the JsIrBuilder subclass for now though.

+  }
 }
 
 /// State shared between JsIrBuilders within the same function.
@@ -2695,6 +2704,11 @@ class JsIrBuilder extends IrBuilder {
     return isNotCheck ? buildNegation(check) : check;
   }
 
+  @override
+  ir.Primitive buildCheckNull(ir.Primitive value) {

[Johnni Winther, 2015/05/29 10:23:24]

Remove this.

[Siggi Cherem (dart-lang), 2015/05/29 18:14:05]

Acknowledged.

+    assert(isOpen);
+    return addPrimitive(new ir.Identical(value, buildNullConstant()));

[Kevin Millikin (Google), 2015/05/29 11:02:03]

This is completely safe, but since buildNullConstant() has the side effect of
emitting part of a CPS term, I normally write:

ir.Primitive right = buildNullConstant();
return addPrimitive(new ir.Identical(value, right));

[Siggi Cherem (dart-lang), 2015/05/29 18:14:05]

Done.

+  }
 }