Reduce strong mode errors and warnings

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

Index: pkg/front_end/lib/src/fasta/builder/builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/builder.dart b/pkg/front_end/lib/src/fasta/builder/builder.dart
index 3424c99a8b63af8a2f5e95721cffce96a8c3f74a..244f1259d27151fe4cefb4bf6c74a2aa2d70beda 100644
--- a/pkg/front_end/lib/src/fasta/builder/builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/builder.dart
@@ -89,16 +89,16 @@ abstract class Builder {
 
   /// Resolve types (lookup names in scope) recorded in this builder and return
   /// the number of types resolved.
-  int resolveTypes(Builder parent) => 0;
+  int resolveTypes(covariant Builder parent) => 0;

[ahe, 2017/02/07 11:03:07]

Do you have to add "covariant" here?

[Siggi Cherem (dart-lang), 2017/02/07 21:59:25]

There are two overrides that strengthen the argument type (in ClassBuilder and
FunctionTypeBuilder). However, both of them used LibraryBuilder as the type.

So we have 3 options:

a- make it LibraryBuilder here.
b- add covariant here.
c- add covariant on each override (the type system allows you to add the
covariant keyword on either the base definition or on each override).

Any preference? It seems like (a) should be fine here, but you know better.

[ahe, 2017/02/08 19:59:23]

I was curious and wasn't sure if c was an option. I'm glad that it is, but
covariant is fine here.

[Siggi Cherem (dart-lang), 2017/02/09 04:44:02]

ok - switched back to covariant. If `LibraryBuilder` is a better match here let
me know.

 
   /// Resolve constructors (lookup names in scope) recorded in this builder and
   /// return the number of constructors resolved.
-  int resolveConstructors(Builder parent) => 0;
+  int resolveConstructors(covariant Builder parent) => 0;
 
   /// Look for methods with the same name as their enclosing class and convert
   /// them to constructors. Return the number of methods converted to
   /// constructors.
-  int convertConstructors(Builder parent) => 0;
+  int convertConstructors(covariant Builder parent) => 0;
 
   /// This builder and [other] has been imported into [library] using [name].
   ///