Use downlevel-dts, test compile in TS 3.4

.github/workflows/test_old_compile.yml

@@ -0,0 +1,29 @@
+name: Build in TSC 3.4
+
+on: [push, pull_request]
+
+
+jobs:
+  build_old:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 12
+
+      - name: NPM install
+        run: npm ci
+
+      - name: Build
+        run: npm run build
+
+      - name: Install typescript@~3.4
+        run: npm install typescript@~3.4
+
+      - name: Print TypeScript version
+        run: npx tsc --version
+
+      - name: Type check .d.ts files
+        run: npx tsc ts3.4/lit-element.d.ts --noEmit -t esnext --moduleResolution Node

.gitignore

@@ -1,5 +1,6 @@
 /node_modules/
 /lib/
+/ts3.4/**/*.d.ts
 
 #Compiler Output
 lit-element.js

.npmignore

@@ -1 +1,10 @@
 docs/
+.travis.yml
+check-version-tracker.js
+rollup.config.js
+tslint.json
+tsconfig_apidoc.json
+wct.conf.json
+.vscode/
+.github/
+.clang-format

package.json

@@ -21,7 +21,7 @@
     "/lit-element.js.map"
   ],
   "scripts": {
-    "build": "tsc",
+    "build": "tsc && downlevel-dts . ts3.4",
     "build:babel-test": "babel src/test/lib/decorators_test.ts --out-file test/lib/decorators-babel_test.js",
     "gen-docs": "typedoc --readme docs/_api/api-readme.md --tsconfig tsconfig_apidoc.json --mode modules --theme docs/_api/theme --excludeNotExported --excludePrivate --ignoreCompilerErrors --exclude '{**/*test*,**/node_modules/**,**/test/**}' --out ./docs/api --gaID UA-39334307-23 src/**/*.ts",
     "test": "npm run build && npm run build:babel-test && wct",
@@ -46,6 +46,7 @@
     "@webcomponents/webcomponentsjs": "^2.2.3",
     "chai": "^4.0.2",
     "clang-format": "^1.2.4",
+    "downlevel-dts": "^0.4.0",
     "lit-element-benchmarks": "^0.1.0",
     "mocha": "^6.2.2",
     "np": "^5.2.1",
@@ -56,12 +57,19 @@
     "tachometer": "^0.4.16",
     "tslint": "^5.20.1",
     "typedoc": "^0.14.2",
-    "typescript": "^3.8.2",
+    "typescript": "~3.8.2",
     "uglify-es": "^3.3.9",
     "wct-mocha": "^1.0.0",
     "web-component-tester": "^6.9.2"
   },
   "typings": "lit-element.d.ts",
+  "typesVersions": {
+    "<3.8": {
+      "*": [
+        "ts3.4/*"
+      ]
+    }
+  },
   "dependencies": {
     "lit-html": "^1.1.1"
   },

ts3.4/tsconfig.json

@@ -0,0 +1,28 @@
+{
+  "compilerOptions": {
+    "target": "es2017",
+    "module": "es2015",
+    "moduleResolution": "node",
+    "lib": ["esnext.array", "esnext", "es2017", "dom"],
+    "declaration": true,
+    "declarationMap": true,
+    "sourceMap": true,
+    "inlineSources": true,
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "noImplicitReturns": true,
+    "noFallthroughCasesInSwitch": true,
+    "noImplicitAny": true,
+    "noImplicitThis": true,
+    "outDir": "./",
+    // Only necessary because @types/uglify-js can't find types for source-map
+    "skipLibCheck": true,
+    "experimentalDecorators": true
+  },
+  "include": [
+    "custom_typings/**/*.ts",
+    "src/**/*.ts"
+  ],
+  "exclude": []
+}