Revert "Removed superfluous copy in license checker (flutter#167146)" (flutter#167246)

This reverts commit b4dc233.

The license script provides a RegExp wrapper that tracks the cost of the
script's regexes. Dart's RegExp.allMatches returns an Iterable whose
iterators lazily execute the regex. Calling toList will execute the
regex on the entire input within allMatches, allowing measurement of the
regex's cost. Returning the Iterable will instead shift the execution
cost to the caller.

Also, in local measurements I'm seeing significantly better performance
from finding all of the matches immediately with toList versus having
the caller use the lazy iterator.

Author: jason-simmons

engine/src/flutter/ci/licenses_golden/tool_signature

@@ -1,2 +1,2 @@
-Signature: bff1faf38fdddc991c42ef472cefd3e7
+Signature: 72079bc1edfa9c01900da12816e18f0e
 

engine/src/flutter/tools/licenses/lib/regexp_debug.dart

@@ -103,7 +103,7 @@ class RegExp implements core.RegExp {
   @override
   Iterable<RegExpMatch> allMatches(String input, [int start = 0]) {
     _stopwatch.start();
-    final Iterable<RegExpMatch> result = _pattern.allMatches(input, start);
+    final List<RegExpMatch> result = _pattern.allMatches(input, start).toList();
     _stopwatch.stop();
     _testCount += 1;
     if (result.isNotEmpty) {