Move to dartdoc 0.20.0 and enable preview-dart-2.

Change-Id: I8e1b7957a416644b8c7d02bee60c248e6c6d9827
Reviewed-on: https://dart-review.googlesource.com/57444
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>

DEPS

@@ -79,7 +79,7 @@ vars = {
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_tag": "1.0.14",  # Please see the note above before updating.
 
-  "dartdoc_tag" : "v0.19.1",
+  "dartdoc_tag" : "v0.20.0",
   "fixnum_tag": "0.10.5",
   "func_rev": "25eec48146a58967d75330075ab376b3838b18a8",
   "glob_tag": "1.1.5",

sdk/bin/dartdoc

@@ -26,4 +26,4 @@ SNAPSHOT="$BIN_DIR/snapshots/dartdoc.dart.snapshot"
 
 # We are running the snapshot in the built SDK.
 DART="$BIN_DIR/dart"
-exec "$DART" --packages="$BIN_DIR/snapshots/resources/dartdoc/.packages" "$SNAPSHOT" "$@"
+exec "$DART" "--preview-dart-2" --packages="$BIN_DIR/snapshots/resources/dartdoc/.packages" "$SNAPSHOT" "$@"

sdk/bin/dartdoc.bat

@@ -14,7 +14,7 @@ for %%i in ("%RETURNED_BIN_DIR%") do set BIN_DIR=%%~fi
 set DART=%BIN_DIR%\dart
 set SNAPSHOT=%BIN_DIR%\snapshots\dartdoc.dart.snapshot
 
-"%DART%" --packages="%BIN_DIR%/snapshots/resources/dartdoc/.packages" "%SNAPSHOT%" %*
+"%DART%" "--preview-dart-2" --packages="%BIN_DIR%/snapshots/resources/dartdoc/.packages" "%SNAPSHOT%" %*
 
 endlocal
 

utils/dartdoc/BUILD.gn

@@ -5,6 +5,7 @@
 import("../application_snapshot.gni")
 
 application_snapshot("dartdoc") {
+  dart_version = 2
   main_dart = "../../third_party/pkg/dartdoc/bin/dartdoc.dart"
   training_args = [ "--help" ]
 }