Fix Fuchsia build (flutter#7105)

Author: tvolkert

packages/flutter_tools/bin/fuchsia_builder.dart

@@ -12,6 +12,7 @@ import '../lib/src/base/config.dart';
 import '../lib/src/base/context.dart';
 import '../lib/src/base/logger.dart';
 import '../lib/src/base/os.dart';
+import '../lib/src/base/process_manager.dart';
 import '../lib/src/cache.dart';
 import '../lib/src/flx.dart';
 import '../lib/src/globals.dart';
@@ -36,6 +37,7 @@ Future<Null> main(List<String> args) async {
   executableContext.setVariable(Logger, new StdoutLogger());
   executableContext.runInZone(() {
     // Initialize the context with some defaults.
+    context.putIfAbsent(ProcessManager, () => new ProcessManager());
     context.putIfAbsent(Logger, () => new StdoutLogger());
     context.putIfAbsent(Cache, () => new Cache());
     context.putIfAbsent(Config, () => new Config());

packages/flutter_tools/lib/executable.dart

@@ -96,6 +96,10 @@ Future<Null> main(List<String> args) async {
   // Make the context current.
   _executableContext.runInZone(() {
     // Initialize the context with some defaults.
+    // NOTE: Similar lists also exist in `bin/fuchsia_builder.dart` and
+    // `test/src/context.dart`. If you update this list of defaults, look
+    // in those locations as well to see if you need a similar update there.
+
     // Seed these context entries first since others depend on them
     context.putIfAbsent(ProcessManager, () => new ProcessManager());
     context.putIfAbsent(Logger, () => new StdoutLogger());