fix: should resolve to full path when setting default entryFiles

closes #3616
closes #3618

packages/@vue/cli-service/__tests__/Service.spec.js

@@ -253,6 +253,7 @@ test('api: configureWebpack', () => {
   }])
 
   const config = service.resolveWebpackConfig()
+  console.log(process.env.VUE_CLI_ENTRY_FILES)
   expect(config.output.path).toBe('test-dist-2')
 })
 
@@ -296,6 +297,23 @@ test('api: configureWebpack preserve ruleNames', () => {
   expect(config.module.rules[0].__ruleNames).toEqual(['js'])
 })
 
+test.only('internal: should correctly set VUE_CLI_ENTRY_FILES', () => {
+  const service = createMockService([{
+    id: 'test',
+    apply: api => {
+      api.configureWebpack(config => {
+        config.entry = {
+          page1: './src/page1.js',
+          page2: './src/page2.js'
+        }
+      })
+    }
+  }])
+
+  service.resolveWebpackConfig()
+  expect(process.env.VUE_CLI_ENTRY_FILES).toEqual('["/src/page1.js","/src/page2.js"]')
+})
+
 test('api: configureDevServer', () => {
   const cb = () => {}
   const service = createMockService([{

packages/@vue/cli-service/lib/Service.js

@@ -270,10 +270,21 @@ module.exports = class Service {
       )
     }
 
-    const entryFiles = Object.values(config.entry || []).reduce((allEntries, curr) => {
-      return allEntries.concat(curr)
-    }, [])
-    process.env.VUE_CLI_ENTRY_FILES = JSON.stringify(entryFiles)
+    if (typeof config.entry !== 'function') {
+      let entryFiles
+      if (typeof config.entry === 'string') {
+        entryFiles = [config.entry]
+      } else if (Array.isArray(config.entry)) {
+        entryFiles = config.entry
+      } else {
+        entryFiles = Object.values(config.entry || []).reduce((allEntries, curr) => {
+          return allEntries.concat(curr)
+        }, [])
+      }
+
+      entryFiles = entryFiles.map(file => path.resolve(this.context, file))
+      process.env.VUE_CLI_ENTRY_FILES = JSON.stringify(entryFiles)
+    }
 
     return config
   }