Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/parser/parser.dart

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index b1f6f876f1c3f2fbd3f698702047c5827f418aed..7efbe326d870c1e075416525f30b53ad855a3a9c 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -2455,7 +2455,7 @@ class Parser {
   Token parseParenthesizedExpression(Token token) {
     // We expect [begin] to be of type [BeginGroupToken], but we don't know for
     // sure until after calling expect.
-    var begin = token;
+    BeginGroupToken begin = token;

[Siggi Cherem (dart-lang), 2017/02/02 04:26:58]

strong mode infers `var` to be the type of the rhs, which is Token.

[ahe, 2017/02/02 07:28:30]

The type should be dynamic. This cast isn't correct. Take a look at the comment
above.

[kasperl, 2017/02/02 07:51:30]

var first = token;
token = expect('(', token);
BeginGroupToken begin = first;

seems cleaner to me. 

[Siggi Cherem (dart-lang), 2017/02/06 23:35:50]

Sorry - I totally missed the comment. Thanks for clarifying. I went ahead with
Kasper's suggestion here.

     token = expect('(', token);
     // [begin] is now known to have type [BeginGroupToken].
     token = parseExpression(token);