GN: Fall back on dart_bootstrap where appropriate.

runtime/observatory/BUILD.gn

Index: runtime/observatory/BUILD.gn
diff --git a/runtime/observatory/BUILD.gn b/runtime/observatory/BUILD.gn
index 5058eed80c26854096b03f541a929d7435b98d50..4978c1ab6b6d374c74d48f160a4cdcbd2a0cebae 100644
--- a/runtime/observatory/BUILD.gn
+++ b/runtime/observatory/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("../../utils/invoke_dart.gni")
+
 # Currently paths here are hard coded for convenience in building Mojo/Flutter.
 declare_args() {
   # Specify the path to a host compatible version of the Dart SDK.
@@ -10,9 +12,40 @@ declare_args() {
 
   # Specify the path to a host compatible version of pub.
   # This is used to compile the Observatory frontend sources.
+  #
+  # The observatory build tool searches for pub first using this GN arg, then
+  # for the checked-in SDK, and if that doesn't work, it uses the dart_bootstrap
+  # built as part of the build to invoke pub. If this arg is set to a non-empty
+  # string, we assume that there is no need for dart_bootstrap. If it is set to
+  # "", then we assume that there may be a need for dart_bootstrap.
   dart_host_pub_exe = rebase_path("$dart_host_sdk/bin/pub")
 }
 
+# Construct arguments to the observatory tool for finding pub.
+pub_build_deps = []
+pub_build_args = []
+if (dart_host_pub_exe != "") {
+  pub_build_args = [
+    "--pub-executable",
+    dart_host_pub_exe,
+  ]
+} else {
+  pub_build_deps += [ "../bin:dart_bootstrap($host_toolchain)" ]
+
+  dart_out_dir =
+      get_label_info("../bin:dart_bootstrap($host_toolchain)", "root_out_dir")
+  dart_bootstrap =
+      rebase_path("$dart_out_dir/dart_bootstrap$dart_executable_suffix")
+
+  pub_build_args = [
+    "--sdk=True",
+    "--dart-executable",
+    dart_bootstrap,
+  ]
+}
+
+current_dir = rebase_path(".", "//")
+
 # Helper build rules for packaging the Dart observatory resources.
 observatory_sources_gypi =
     exec_script("../../tools/gypi_to_gn.py",
@@ -28,30 +61,25 @@ copy("copy_observatory") {
 }
 
 action("write_observatory_pubspec_yaml") {
-  deps = [
-    ":copy_observatory",
-  ]
-
-  script = "../../tools/observatory_tool.py"
+  deps = [ ":copy_observatory" ] + pub_build_deps
 
   inputs = [
     rebase_path("pubspec.yaml"),
   ]
 
-  current_dir = rebase_path(".", "//")
-  args = [
-    "--silent=True",
-    "--pub-executable",
-    dart_host_pub_exe,
-    "--directory",
-    rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
-    "--command",
-    "rewrite",
-    rebase_path("../observatory/pubspec.yaml"),
-    rebase_path("$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml"),
-    "../../third_party/",
-    rebase_path("../../third_party/"),
-  ]
+  script = "../../tools/observatory_tool.py"
+  args =
+      pub_build_args + [
+        "--silent=True",
+        "--directory",
+        rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
+        "--command",
+        "rewrite",
+        rebase_path("../observatory/pubspec.yaml"),
+        rebase_path("$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml"),
+        "../../third_party/",
+        rebase_path("../../third_party/"),
+      ]
 
   outputs = [
     "$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml",
@@ -59,29 +87,23 @@ action("write_observatory_pubspec_yaml") {
 }
 
 action("copy_observatory_deps") {
-  deps = [
-    ":write_observatory_pubspec_yaml",
-  ]
-
-  script = "../../tools/observatory_tool.py"
+  deps = [ ":write_observatory_pubspec_yaml" ] + pub_build_deps
 
-  current_dir = rebase_path(".", "//")
   inputs = [
-    script,
     "$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml",
   ]
 
-  args = [
-    "--silent=True",
-    "--pub-executable",
-    dart_host_pub_exe,
-    "--stamp",
-    rebase_path("$root_gen_dir/observatory_copy/$current_dir/packages.stamp"),
-    "--directory",
-    rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
-    "--command",
-    "get",
-  ]
+  script = "../../tools/observatory_tool.py"
+  args = pub_build_args + [
+           "--silent=True",
+           "--stamp",
+           rebase_path(
+               "$root_gen_dir/observatory_copy/$current_dir/packages.stamp"),
+           "--directory",
+           rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
+           "--command",
+           "get",
+         ]
 
   outputs = [
     "$root_gen_dir/observatory_copy/$current_dir/packages.stamp",
@@ -89,34 +111,27 @@ action("copy_observatory_deps") {
 }
 
 action("pub_build_observatory") {
-  current_dir = rebase_path(".", "//")
+  deps = [
+           ":copy_observatory",
+           ":copy_observatory_deps",
+         ] + pub_build_deps
+
   sources = rebase_path(observatory_sources_gypi.sources,
                         "",
                         "$root_gen_dir/observatory_copy/$current_dir")
 
-  deps = [
-    ":copy_observatory",
-    ":copy_observatory_deps",
-  ]
+  inputs =
+      sources + [ "$root_gen_dir/observatory_copy/$current_dir/packages.stamp" ]
 
   script = "../../tools/observatory_tool.py"
-
-  inputs = [
-    script,
-    "$root_gen_dir/observatory_copy/$current_dir/packages.stamp",
-  ]
-  inputs += sources
-
-  args = [
-    "--silent=True",
-    "--pub-executable",
-    dart_host_pub_exe,
-    "--directory",
-    rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
-    "--command",
-    "build",
-    rebase_path("$root_out_dir/observatory/build"),
-  ]
+  args = pub_build_args + [
+           "--silent=True",
+           "--directory",
+           rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
+           "--command",
+           "build",
+           rebase_path("$root_out_dir/observatory/build"),
+         ]
 
   outputs = [
     "$root_out_dir/observatory/build/web/main.dart.js",
@@ -124,9 +139,7 @@ action("pub_build_observatory") {
 }
 
 action("deploy_observatory") {
-  deps = [
-    ":pub_build_observatory",
-  ]
+  deps = [ ":pub_build_observatory" ] + pub_build_deps
 
   script = "../../tools/observatory_tool.py"
 
@@ -135,15 +148,13 @@ action("deploy_observatory") {
     "$root_out_dir/observatory/build/web/main.dart.js",
   ]
 
-  args = [
-    "--silent=True",
-    "--pub-executable",
-    dart_host_pub_exe,
-    "--directory",
-    rebase_path("$root_out_dir/observatory"),
-    "--command",
-    "deploy",
-  ]
+  args = pub_build_args + [
+           "--silent=True",
+           "--directory",
+           rebase_path("$root_out_dir/observatory"),
+           "--command",
+           "deploy",
+         ]
 
   outputs = [
     "$root_out_dir/observatory/deployed/web/main.dart.js",
@@ -164,16 +175,15 @@ template("observatory_archive") {
       ":deploy_observatory",
     ]
 
-    script = "../tools/create_archive.py"
-
     inputs = [
-      script,
       "$root_out_dir/observatory/deployed/web/main.dart.js",
     ]
 
     inner_namespace = invoker.inner_namespace
     outer_namespace = invoker.outer_namespace
     output_name = target_name
+
+    script = "../tools/create_archive.py"
     args = [
       "--output",
       rebase_path("$root_gen_dir/observatory/${output_name}.cc"),