Fixed missing implementations of toString() in two tests (#21509)

packages/flutter/test/material/theme_test.dart

@@ -459,6 +459,10 @@ class _TextStyleProxy implements TextStyle {
   @override TextBaseline get textBaseline => _delegate.textBaseline;
   @override double get wordSpacing => _delegate.wordSpacing;
 
+  @override
+  String toString({DiagnosticLevel minLevel = DiagnosticLevel.debug}) =>
+      super.toString();
+
   @override
   DiagnosticsNode toDiagnosticsNode({String name, DiagnosticsTreeStyle style}) {
     throw new UnimplementedError();

packages/flutter/test/rendering/proxy_box_test.dart

@@ -289,4 +289,7 @@ class _FakeTicker implements Ticker {
 
   @override
   void unscheduleTick() {}
+
+  @override
+  String toString({bool debugIncludeStack = false}) => super.toString();
 }