Fix invariant breakage on erroneous constant variables.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 3a1e981f5b85a870f330bb945e24f3e01b9ed29d..3c2fceef7e2276f3f67482a4b37eda44d41c1e12 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -1408,11 +1408,17 @@ abstract class ConstantVariableMixin implements VariableElement {
       originVariable.constant = value;
       return;
     }
+    if (constantCache != null &&
+        constantCache.kind == ConstantExpressionKind.ERRONEOUS) {
+      // TODO(johnniwinther): Find out why we sometimes compute a non-erroneous
+      // constant for a variable already known to be erroneous.
+      return;
+    }
     assert(invariant(this, constantCache == null || constantCache == value,
         message: "Constant has already been computed for $this. "
             "Existing constant: "
-            "${constantCache != null ? constantCache.toDartText() : ''}, "
-            "New constant: ${value != null ? value.toDartText() : ''}."));
+            "${constantCache != null ? constantCache.toStructuredText() : ''}, "
+            "New constant: ${value != null ? value.toStructuredText() : ''}."));
     constantCache = value;
   }
 }