Avoid use of NonConstantValue in SSA

BUG=

Review URL: https://codereview.chromium.org/1934553002 .

Author: johnniwinther

pkg/compiler/lib/src/constants/expressions.dart

@@ -1450,8 +1450,7 @@ class DeferredConstantExpression extends ConstantExpression {
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
     return new DeferredConstantValue(
-        expression.evaluate(environment, constantSystem),
-        prefix);
+        expression.evaluate(environment, constantSystem), prefix);
   }
 
   @override

pkg/compiler/lib/src/constants/values.dart

@@ -159,7 +159,7 @@ class NullConstantValue extends PrimitiveConstantValue {
   /** The value a Dart null is compiled to in JavaScript. */
   static const String JsNull = "null";
 
-  factory NullConstantValue() => const NullConstantValue._internal();
+  const factory NullConstantValue() = NullConstantValue._internal;
 
   const NullConstantValue._internal();
 

pkg/compiler/lib/src/deferred_load.dart

@@ -9,9 +9,7 @@ import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'constants/expressions.dart'
-    show
-        ConstantExpression,
-        ConstantExpressionKind;
+    show ConstantExpression, ConstantExpressionKind;
 import 'constants/values.dart'
     show
         ConstantValue,
@@ -338,12 +336,13 @@ class DeferredLoadTask extends CompilerTask {
         TreeElements treeElements = analyzableElement.resolvedAst.elements;
         assert(treeElements != null);
 
-        treeElements.forEachConstantNode(
-            (Node node, ConstantExpression expression) {
+        treeElements
+            .forEachConstantNode((Node node, ConstantExpression expression) {
           // Explicitly depend on the backend constants.
           ConstantValue value = backend.constants.getConstantValue(expression);
           assert(invariant(node, value != null,
-              message: "No constant value for ${expression.toStructuredText()}."));
+              message:
+                  "No constant value for ${expression.toStructuredText()}."));
           constants.add(value);
         });
       }

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

@@ -198,6 +198,10 @@ class HGraph {
     HConstant result = constants[constant];
     // TODO(johnniwinther): Support source information per constant reference.
     if (result == null) {
+      if (!constant.isConstant) {
+        // We use `null` as the value for invalid constant expressions.
+        constant = const NullConstantValue();
+      }
       TypeMask type = computeTypeMask(compiler, constant);
       result = new HConstant.internal(constant, type)
         ..sourceInformation = sourceInformation;