Dart 2 fixes

• Breaking change: Generated RpcClient stubs use the generic invoke method. Requires package:protobuf version 0.8.0 or newer.
• Dart 2 fixes.

Dart 2 fix

Sync from internal repo. (#101)

* Dart 2 fix.

More Dart 2

• Small performance tweak for DDC.

Performance and Dart 2.0

• Add fast getters for common types.
• Only pass index instead of tag and index in generated code.
• Fix uses-dynamic-as-bottom error in generated gRPC code.

Enum values

• Added enumValues to FieldInfo.

Clashes and stubs

• Avoid name clashes between import prefix and field names.
• Avoid name clashes between generated enum and extension class names.
• Updated gRPC client stub generation to match latest changes to dart-lang/grpc-dart.

gRPC client stub tweaks

• Updated gRPC client stub generation to produce code matching latest changes to dart-lang/dart-grpc.

Prepare for Dart 2

• Use real generic syntax instead of comment-based.
• Support 2.0.0 dev SDKs.

gRPC support

• Added call options to gRPC client stubs.

gRPC support

• Added gRPC stub generation.
• Updated descriptor.proto from google/protobuf v3.3.0.