Narrow regexp for import search in bot's analyze script. (#26539)

This was causing analysis to fail when there was an import statement in a comment, such as when snippets add imports to their examples.

I narrowed the RegExp to match only those lines which aren't commented out, but it really should probably be using the analysis server to catch all cases (e.g. if someone put the doc comment into /** */ comments, it could still match). Since this is a Flutter-specific script, it's probably not worth doing that.

dev/bots/analyze.dart

@@ -383,8 +383,8 @@ bool _matches<T>(List<T> a, List<T> b) {
   return true;
 }
 
-final RegExp _importPattern = RegExp(r"import 'package:flutter/([^.]+)\.dart'");
-final RegExp _importMetaPattern = RegExp(r"import 'package:meta/meta.dart'");
+final RegExp _importPattern = RegExp(r'''^\s*import (['"])package:flutter/([^.]+)\.dart\1''');
+final RegExp _importMetaPattern = RegExp(r'''^\s*import (['"])package:meta/meta.dart\1''');
 
 Set<String> _findDependencies(String srcPath, List<String> errors, { bool checkForMeta = false }) {
   return Directory(srcPath).listSync(recursive: true).where((FileSystemEntity entity) {
@@ -395,7 +395,7 @@ Set<String> _findDependencies(String srcPath, List<String> errors, { bool checkF
     for (String line in file.readAsLinesSync()) {
       Match match = _importPattern.firstMatch(line);
       if (match != null)
-        result.add(match.group(1));
+        result.add(match.group(2));
       if (checkForMeta) {
         match = _importMetaPattern.firstMatch(line);
         if (match != null) {