Rewrite Constantifier.visitLet

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

Index: pkg/compiler/lib/src/kernel/element_map.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map.dart b/pkg/compiler/lib/src/kernel/element_map.dart
index da2fb5f80f9f9511f6868783290a3dfc97f1877a..321d88f2147e52bcfde427e4d5d531d83d216e22 100644
--- a/pkg/compiler/lib/src/kernel/element_map.dart
+++ b/pkg/compiler/lib/src/kernel/element_map.dart
@@ -721,22 +721,26 @@ class Constantifier extends ir.ExpressionVisitor<ConstantExpression> {
 
   @override
   ConstantExpression visitLet(ir.Let node) {
-    if (node.body is ir.ConditionalExpression) {
-      ir.ConditionalExpression conditional = node.body;
-      if (conditional.condition is ir.MethodInvocation) {
-        ir.MethodInvocation methodInvocation = conditional.condition;
-        if (methodInvocation.name.name == BinaryOperator.EQ.name &&
-            methodInvocation.receiver is ir.VariableGet &&
-            methodInvocation.arguments.positional.single is ir.NullLiteral &&
-            conditional.otherwise is ir.VariableGet) {
-          ir.VariableGet variableGet1 = methodInvocation.receiver;
-          ir.VariableGet variableGet2 = conditional.otherwise;
-          if (variableGet1.variable == node.variable &&
-              variableGet2.variable == node.variable) {
+    ir.Expression body = node.body;
+    if (body is ir.ConditionalExpression) {
+      ir.Expression condition = body.condition;
+      if (condition is ir.MethodInvocation) {
+        ir.Expression receiver = condition.receiver;
+        ir.Expression otherwise = body.otherwise;
+        if (condition.name.name == BinaryOperator.EQ.name &&
+            receiver is ir.VariableGet &&
+            condition.arguments.positional.single is ir.NullLiteral &&
+            otherwise is ir.VariableGet) {
+          if (receiver.variable == node.variable &&
+              otherwise.variable == node.variable) {
             // We have <left> ?? <right> encoded as:
             //    let #1 = <left> in #1 == null ? <right> : #1
             ConstantExpression left = visit(node.variable.initializer);
-            ConstantExpression right = visit(conditional.then);
+            ConstantExpression right = visit(body.then);
+            // TODO(johnniwinther): Remove [IF_NULL] binary constant expression
+            // when the resolver is removed; then we no longer need the
+            // expressions to be structurally equivalence for equivalence
+            // testing.
             return new BinaryConstantExpression(
                 left, BinaryOperator.IF_NULL, right);
           }