Use VMScriptRef to identify VM scripts, not URI

Previously, coverage collection assumed that a script could be uniquely
identified by URI, which is not a valid assumption. For example, when a
part is loaded via two libraries, both of which are loaded by an
isolate, the VM will track these are two scripts that map to the same
URI.

During collection, we now track each script by its (unique)
VMScriptRef. This ensures we lookup the correct script when computing
the affected line for each hit token. The hitmap remains URI based,
since in the end, we want a single, unified set of line->hitCount
mappings per script.

Fixes #194

lib/src/collect.dart

@@ -80,10 +80,9 @@ Future _waitIsolatesPaused(VMServiceClient service, {Duration timeout}) async {
 Future<List<Map<String, dynamic>>> _getCoverageJson(
     VMServiceClient service, VMSourceReport report) async {
   var scriptRefs = report.ranges.map((r) => r.script).toSet();
-  var scripts = <Uri, VMScript>{};
-  for (var script in await Future.wait<VMScript>(
-      scriptRefs.map((ref) => ref.load()).toList())) {
-    scripts[script.uri] = script;
+  var scripts = <VMScriptRef, VMScript>{};
+  for (var ref in scriptRefs) {
+    scripts[ref] = await ref.load();
   }
 
   // script uri -> { line -> hit count }
@@ -94,7 +93,7 @@ Future<List<Map<String, dynamic>>> _getCoverageJson(
 
     hitMaps.putIfAbsent(range.script.uri, () => <int, int>{});
     var hitMap = hitMaps[range.script.uri];
-    var script = scripts[range.script.uri];
+    var script = scripts[range.script];
     for (VMScriptToken hit in range.hits ?? []) {
       var line = script.sourceLocation(hit).line + 1;
       hitMap[line] = hitMap.containsKey(line) ? hitMap[line] + 1 : 1;
@@ -108,27 +107,26 @@ Future<List<Map<String, dynamic>>> _getCoverageJson(
   // Output JSON
   var coverage = <Map<String, dynamic>>[];
   hitMaps.forEach((uri, hitMap) {
-    var script = scripts[uri];
-    coverage.add(_toScriptCoverageJson(script, hitMap));
+    coverage.add(_toScriptCoverageJson(uri, hitMap));
   });
   return coverage;
 }
 
 /// Returns a JSON hit map backward-compatible with pre-1.16.0 SDKs.
 Map<String, dynamic> _toScriptCoverageJson(
-    VMScript script, Map<int, int> hitMap) {
+    Uri scriptUri, Map<int, int> hitMap) {
   var json = <String, dynamic>{};
   var hits = <int>[];
   hitMap.forEach((line, hitCount) {
     hits.add(line);
     hits.add(hitCount);
   });
-  json['source'] = '${script.uri}';
+  json['source'] = '$scriptUri';
   json['script'] = {
     'type': '@Script',
     'fixedId': true,
-    'id': 'libraries/1/scripts/${Uri.encodeComponent(script.uri.toString())}',
-    'uri': '${script.uri}',
+    'id': 'libraries/1/scripts/${Uri.encodeComponent(scriptUri.toString())}',
+    'uri': '$scriptUri',
     '_kind': 'library',
   };
   json['hits'] = hits;