Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/kernel/builder_accessors.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.builder_accessors;
 
 export 'frontend_accessors.dart' show
     wrapInvalid;
 
 import 'frontend_accessors.dart' show
@@ skipping 79 matching lines @@
     return throwNoSuchMethodError(plainNameForRead, new Arguments.empty(), uri,
         charOffset, coreTypes, isGetter: true);
   }
 
   Expression makeInvalidWrite(Expression value) {
     return throwNoSuchMethodError(plainNameForWrite,
         new Arguments(<Expression>[value]), uri, charOffset, coreTypes,
         isSetter: true);
   }
 
-  Expression doInvocation(int charOffset, Arguments arguments);
+  TreeNode doInvocation(int charOffset, Arguments arguments);
 
   buildPropertyAccess(IncompleteSend send, bool isNullAware) {
     if (send is SendAccessor) {
       return buildMethodInvocation(buildSimpleRead(), send.name, send.arguments,
           charOffset, isNullAware: isNullAware);
     } else {
       return PropertyAccessor.make(helper, charOffset, buildSimpleRead(),
           send.name, null, null, isNullAware);
     }
   }
@@ skipping 610 matching lines @@
             buildIsNull(new VariableGet(variable)),
             new NullLiteral(),
             new MethodInvocation(new VariableGet(variable), name, arguments)
                 ..fileOffset = charOffset,
             const DynamicType()));
   } else {
     return new MethodInvocation(receiver, name, arguments)
         ..fileOffset = charOffset;
   }
 }