perfetto_protos_dart_gen is not hermetic

[rmacnak-google]

$ ./tools/build.py all
[1/4] ACTION //runtime/vm:perfetto_protos_dart_gen(//build/toolchain/mac:clang_arm64)
FAILED: gen/pkg/vm_service_protos/lib/src/protos/perfetto/common/builtin_clock.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/clock_snapshot.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/interned_data/interned_data.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/profiling/profile_common.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/profiling/profile_packet.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/trace.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/trace_packet.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/track_event/debug_annotation.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/track_event/process_descriptor.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/track_event/thread_descriptor.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/track_event/track_descriptor.pb.dart gen/pkg/vm_service_protos/lib/src/protos/perfetto/trace/track_event/track_event.pb.dart 
python3 ../../build/secondary/third_party/protobuf/protoc_wrapper.py protos/perfetto/common/builtin_clock.proto protos/perfetto/trace/clock_snapshot.proto protos/perfetto/trace/interned_data/interned_data.proto protos/perfetto/trace/profiling/profile_common.proto protos/perfetto/trace/profiling/profile_packet.proto protos/perfetto/trace/trace.proto protos/perfetto/trace/trace_packet.proto protos/perfetto/trace/track_event/debug_annotation.proto protos/perfetto/trace/track_event/process_descriptor.proto protos/perfetto/trace/track_event/thread_descriptor.proto protos/perfetto/trace/track_event/track_descriptor.proto protos/perfetto/trace/track_event/track_event.proto --depfile gen/runtime/vm/perfetto_protos_dart_gen.d --depfile-outputs gen/runtime/vm/perfetto_protos_dart_protoc_outputs.protoc_output_info --protoc ./protoc --proto-in-dir ../../runtime/vm --plugin ../../third_party/pkg/protobuf/protoc_plugin/bin/protoc-gen-dart --plugin-out-dir gen/pkg/vm_service_protos/lib/src
protos/perfetto/trace/interned_data/interned_data.proto:27:1: warning: Import protos/perfetto/trace/track_event/debug_annotation.proto is unused.
protos/perfetto/trace/interned_data/interned_data.proto:28:1: warning: Import protos/perfetto/trace/track_event/track_event.proto is unused.
../../third_party/pkg/protobuf/protoc_plugin/bin/protoc-gen-dart: line 3: dart: command not found
--plugin_out: protoc-gen-plugin: Plugin failed with status code 127.
Protoc has returned non-zero status: 1 .
ninja: build stopped: cannot make progress due to previous errors.

This build step attempts to use dart from the PATH. Any use of dart should be either the one created by the build or the prebuilt in tools/sdks/dart-sdk.

@derekxu16