Remove special casing of == null

This avoid reversing the operands for `null == x`
- and make Hello World! fully equivalent.

R=sigmund@google.com

Review-Url: https://codereview.chromium.org/2954123002 .

pkg/compiler/lib/src/ssa/builder_kernel.dart

@@ -719,12 +719,6 @@ class KernelSsaGraphBuilder extends ir.Visitor
     graph.finalize();
   }
 
-  /// Pushes a boolean checking [expression] against null.
-  pushCheckNull(HInstruction expression) {
-    push(new HIdentity(expression, graph.addConstantNull(closedWorld), null,
-        commonMasks.boolType));
-  }
-
   @override
   void defaultExpression(ir.Expression expression) {
     // TODO(het): This is only to get tests working.
@@ -2888,9 +2882,6 @@ class KernelSsaGraphBuilder extends ir.Visitor
   // TODO(het): Decide when to inline
   @override
   void visitMethodInvocation(ir.MethodInvocation invocation) {
-    // Handle `x == null` specially. When these come from null-aware operators,
-    // there is no mapping in the astAdapter.
-    if (_handleEqualsNull(invocation)) return;
     invocation.receiver.accept(this);
     HInstruction receiver = pop();
     Selector selector = _elementMap.getSelector(invocation);
@@ -2901,27 +2892,6 @@ class KernelSsaGraphBuilder extends ir.Visitor
             _visitArgumentsForDynamicTarget(selector, invocation.arguments)));
   }
 
-  bool _handleEqualsNull(ir.MethodInvocation invocation) {
-    if (invocation.name.name == '==') {
-      ir.Arguments arguments = invocation.arguments;
-      if (arguments.types.isEmpty &&
-          arguments.positional.length == 1 &&
-          arguments.named.isEmpty) {
-        bool finish(ir.Expression comparand) {
-          comparand.accept(this);
-          pushCheckNull(pop());
-          return true;
-        }
-
-        ir.Expression receiver = invocation.receiver;
-        ir.Expression argument = arguments.positional.first;
-        if (argument is ir.NullLiteral) return finish(receiver);
-        if (receiver is ir.NullLiteral) return finish(argument);
-      }
-    }
-    return false;
-  }
-
   HInterceptor _interceptorFor(HInstruction intercepted) {
     HInterceptor interceptor =
         new HInterceptor(intercepted, commonMasks.nonNullType);

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

@@ -580,7 +580,12 @@ class KernelAstTypeInferenceMap implements KernelToTypeInferenceMap {
     if (send.name.name == '[]=') {
       return closedWorld.commonMasks.dynamicType;
     }
-    return _resultOf(_target).typeOfSend(_astAdapter.getNode(send));
+    ast.Node node = _astAdapter.getNodeOrNull(send);
+    if (node == null) {
+      assert(send.name.name == '==');
+      return closedWorld.commonMasks.dynamicType;
+    }
+    return _resultOf(_target).typeOfSend(node);
   }
 
   TypeMask typeOfGet(ir.PropertyGet getter) {

tests/compiler/dart2js/kernel/compile_from_dill_test.dart

@@ -49,10 +49,17 @@ main() {
   new Class('');
   Class.staticField;
   var x = null;
+  var y1 = x == null;
+  var y2 = null == x;
+  var z1 = x?.toString();
+  var z2 = x ?? y1;
+  var z3 = x ??= y2;
+  var w = x == null ? null : x.toString();
   for (int i = 0; i < 10; i++) {
     x = i;
     if (i == 5) break;
   }
+  print(x);
   return x;
 }
 '''