Change : to = for default values in pkg.

Leaves some in parser test:
 pkg/front_end/parser_testcases/error_recovery/keyword_named_formal_parameter_prime.dart

TEST=Refactoring, covered by existing tests.

Change-Id: I7a83ef95df3cbd283878b3685b5c747bd89a1b16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/256125
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>

pkg/compiler/test/codegen/model_data/effectively_constant_state.dart

@@ -29,7 +29,7 @@ class Class {
   /*member: Class.state2:elided*/
   final Enum state2;
 
-  Class({this.state1: 1, this.state2: Enum.c});
+  Class({this.state1 = 1, this.state2 = Enum.c});
 
   /*member: Class.method1a:calls=[tester2(0)],params=0*/
   @pragma('dart2js:noInline')