fix: reset coverage result converters between Chrome session runs

Author: tmsns

.changeset/breezy-crabs-watch.md

@@ -0,0 +1,5 @@
+---
+'@web/test-runner-coverage-v8': patch
+---
+
+fix: reset coverage result converters between Chrome session runs

packages/test-runner-coverage-v8/src/index.ts

@@ -85,7 +85,20 @@ export async function v8ToIstanbul(
           if (!cachedConverter) {
             await converter.load();
             cachedConverters.set(filePath, converter);
-          }
+          } else {
+            // When we reuse a cached converter, we need to reset it before using its `applyCoverage` function.
+            // If we don't, the coverage results will be poisoned with the results of the previous uses.
+            // 
+            // This "workaround" is resetting some internal variables of the `V8ToIstanbul` class: `branches` & `functions`.
+            // This can break when newer versions of v8-to-istanbul are released. (variable renaming, more variables are used, ...)
+            //
+            // TODO: use a (stable) clone technique instead when available (`structuredClone` is available in node 17)
+            //
+            // @ts-ignore
+            converter.branches = {};
+            // @ts-ignore
+            converter.functions = {};
+          }  
 
           converter.applyCoverage(entry.functions);
           Object.assign(istanbulCoverage, converter.toIstanbul());