Improve error reporting when a mixin application is invalid.

pkg/analyzer/lib/src/generated/error_verifier.dart

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 21aadca68e5259dfdcc6522001b1cfd6238471c7..4ff01d037398ff74d51b9ed7bc8cdc53e77fd05e 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -3996,6 +3996,10 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
    * See [CompileTimeErrorCode.NO_DEFAULT_SUPER_CONSTRUCTOR_IMPLICIT].
    */
   bool _checkForNoDefaultSuperConstructorImplicit(ClassDeclaration node) {
+    // do nothing if mixin errors have already been reported for this class.
+    if (_enclosingClass.mixinErrorsReported) {
+      return false;
+    }
     // do nothing if there is explicit constructor
     List<ConstructorElement> constructors = _enclosingClass.constructors;
     if (!constructors[0].isSynthetic) {
@@ -4835,6 +4839,13 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
    * [StaticWarningCode.NO_DEFAULT_SUPER_CONSTRUCTOR_EXPLICIT].
    */
   bool _checkForUndefinedConstructorInInitializerImplicit(ConstructorDeclaration node) {
+    if (_enclosingClass == null) {
+      return false;
+    }
+    // do nothing if mixin errors have already been reported for this class.
+    if (_enclosingClass.mixinErrorsReported) {
+      return false;
+    }
     //
     // Ignore if the constructor is not generative.
     //
@@ -4853,9 +4864,6 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
     //
     // Check to see whether the superclass has a non-factory unnamed constructor.
     //
-    if (_enclosingClass == null) {
-      return false;
-    }
     InterfaceType superType = _enclosingClass.supertype;
     if (superType == null) {
       return false;