Handle fields with initializers in constant constructors.

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 e2588fc24fb1a419e139ce03e57f2399ec836445..bb331d96027f82fb2e26338ecc1b4130fdc95288 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -1425,11 +1425,22 @@ abstract class ConstantVariableMixin implements VariableElement {
       // 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.toStructuredText() : ''}, "
-            "New constant: ${value != null ? value.toStructuredText() : ''}."));
+    if (constantCache != null && constantCache != value) {
+      // Allow setting the constant as erroneous. Constants computed during
+      // resolution are locally valid but might be effectively erroneous. For
+      // instance `a ? true : false` where a is `const a = m()`. Since `a` is
+      // declared to be constant, the conditional is assumed valid, but when
+      // computing the value we see that it isn't.
+      // TODO(johnniwinther): Remove this exception when all constant
+      // expressions are computed during resolution.
+      assert(invariant(
+          this, value == null || value.kind == ConstantExpressionKind.ERRONEOUS,
+          message: "Constant has already been computed for $this. "
+              "Existing constant: "
+              "${constantCache != null ? constantCache.toStructuredText() : ''}"
+              ", New constant: "
+              "${value != null ? value.toStructuredText() : ''}."));
+    }
     constantCache = value;
   }
 }