Run benchmarks on GitHub actions.

.github/workflows/benchmarks.yml

@@ -13,35 +13,40 @@ jobs:
 
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-
       - uses: actions/setup-node@v2
         with:
           node-version: 14
 
-      - name: Restore node_modules
+      - uses: actions/checkout@v2
+
+      # By default, the actions/checkout@v2 only retrieves the one commit
+      # associated with the pull request. The benchmarks test against the
+      # `master` branch also, so we explicitly retrieve it here.
+      - name: Fetch additional refs
+        run: git fetch --no-tags --depth=1 origin master
+
+      - name: Restore benchmarks/node_modules
         id: cache
         uses: actions/cache@v2
         with:
-          path: node_modules
-          # Bump the number after `lit-element-1.x-node-modules` to force a
-          # cache update. Note there are multiple cache actions in `tests.yml`
-          # that should all keep keys in sync.
-          key: lit-element-1.x-node-modules-0-${{ runner.os }}-${{ hashFiles('package-lock.json') }}
+          path: benchmarks/node_modules
+          # Bump the number after `lit-element-2.x-benchmarks-node-modules` to
+          # force a cache update. Note there are cache actions in other jobs in
+          # this file that should all keep keys in sync.
+          key: lit-element-2.x-benchmarks-node-modules-0-${{ runner.os }}-${{ hashFiles('benchmarks/package-lock.json') }}
 
       - name: NPM install
         if: steps.cache.outputs.cache-hit != 'true'
+        working-directory: ./benchmarks
         run: npm ci
 
-      - name: Build
-        run: npm run build
-
       - name: Benchmark
+        working-directory: ./benchmarks
         run: npx tach --config tachometer.json --json-file results.json
 
       - name: Report
         uses: andrewiggins/tachometer-reporter-action@v2
         with:
-          path: results.json
+          path: ./benchmarks/results.json
           pr-bench-name: this-change
           base-bench-name: master

benchmarks/.gitignore

@@ -0,0 +1 @@
+/node_modules