Make old code excerpter ignore new <?code-excerpt?> blocks (#2234)

... w/o the need for <!-- skip --> comments. Contributes to #2233

Author: chalin

src/docs/development/ui/layout/tutorial.md

@@ -204,7 +204,6 @@ Since the code for building each row is almost identical, create a private
 helper method named `buildButtonColumn()`, which takes a color, an Icon and
 Text, and returns a column with its widgets painted in the given color.
 
-<!-- skip -->
 <?code-excerpt "lib/main.dart (_buildButtonColumn)" title?>
 ```dart
 class MyApp extends StatelessWidget {
@@ -245,7 +244,6 @@ using `MainAxisAlignment.spaceEvenly` to arrange the free space evenly before,
 between, and after each column. Add the following code just below the
 `titleSection` declaration inside the `build()` method:
 
-<!-- skip -->
 <?code-excerpt "lib/main.dart (buttonSection)" title?>
 ```dart
 Color color = Theme.of(context).primaryColor;
@@ -293,7 +291,6 @@ Define the text section as a variable. Put the text in a Container and add
 padding along each edge. Add the following code just below the `buttonSection`
 declaration:
 
-<!-- skip -->
 <?code-excerpt "lib/main.dart (textSection)" title?>
 ```dart
 Widget textSection = Container(

src/docs/get-started/codelab.md

@@ -210,7 +210,6 @@ packages, on [the Package site](https://pub.dartlang.org/flutter).
 
  3. In `lib/main.dart`, import the new package:
 
-    <!-- skip -->
     <?code-excerpt path-base="codelabs/startup_namer/step2_use_package"?>
     <?code-excerpt "lib/main.dart" title retain="/^import/" replace="/import.*?english.*/[!$&!]/g" indent-by="2"?>
     ```dart
@@ -299,7 +298,6 @@ a child inside the existing `MyApp` stateless widget.
  1. Create a minimal state class. Add the following to the bottom
     of `main.dart`:
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart (RandomWordsState)" title region="RWS-class-only" plaster="// TODO Add build() method" indent-by="2"?>
     ```dart
       class RandomWordsState extends State<RandomWords> {
@@ -322,7 +320,6 @@ a child inside the existing `MyApp` stateless widget.
  2. Add the stateful `RandomWords` widget to `main.dart`.
     The `RandomWords` widget does little else beside creating its State class:
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart (RandomWords)" title indent-by="2"?>
     ```dart
       class RandomWords extends StatefulWidget {
@@ -338,7 +335,6 @@ a child inside the existing `MyApp` stateless widget.
 
  3. Add the `build()` method to `RandomWordsState`:
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart (RandomWordsState)" title indent-by="2" replace="/(\n  )(.*)/$1[!$2!]/g"?>
     ```dart
       class RandomWordsState extends State<RandomWords> {
@@ -418,7 +414,6 @@ lazily, on demand.
     class for saving suggested word pairings.
     Also, add a `_biggerFont` variable for making the font size larger.
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart" title region="RWS-var" indent-by="2" replace="/final .*/[!$&!]/g"?>
     ```dart
       class RandomWordsState extends State<RandomWords> {
@@ -448,7 +443,6 @@ lazily, on demand.
 
  2. Add a `_buildSuggestions()` function to the `RandomWordsState` class:
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart (_buildSuggestions)" title indent-by="2"?>
     ```dart
       Widget _buildSuggestions() {
@@ -486,7 +480,6 @@ lazily, on demand.
 
  3. Add a `_buildRow()` function to `RandomWordsState`:
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart (_buildRow)" title indent-by="2"?>
     ```dart
       Widget _buildRow(WordPair pair) {
@@ -506,7 +499,6 @@ lazily, on demand.
     implements the basic Material Design visual layout.)
     Replace the method body with the highlighted code:
 
-    <!-- skip -->
     <?code-excerpt "lib/main.dart (build)" title region="RWS-build" replace="/(\n  )(return.*|  .*|\);)/$1[!$2!]/g" indent-by="2"?>
     ```dart
       @override

tool/extract.dart

@@ -67,7 +67,7 @@ int _processFile(File file) {
       }
       lastComment = lastComment.substring(0, lastComment.length - 3);
     } else if (lines[index].contains('<?code-') || lines[index].trim().isEmpty) {
-      // Carry on. In particular, don't reset lastComment.
+      lastComment = 'skip';
     } else {
       lastComment = null;
     }