Support Java 1.8 (#25470)

packages/flutter_tools/gradle/flutter.gradle

diff --git a/packages/flutter_tools/gradle/flutter.gradle b/packages/flutter_tools/gradle/flutter.gradle
index e292b18..3cd58a2 100644
--- a/packages/flutter_tools/gradle/flutter.gradle
+++ b/packages/flutter_tools/gradle/flutter.gradle
@@ -26,6 +26,13 @@
     }
 }
 
+android {
+    compileOptions {
+        sourceCompatibility 1.8
+        targetCompatibility 1.8
+    }
+}
+
 apply plugin: FlutterPlugin
 
 class FlutterPlugin implements Plugin<Project> {