build_tools_version是 ext 定义的吗？

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    ext.support_version = "26"
    ext.build_tools_version = "27.0.3"
    ext.min_sdk_version = 24


    repositories {
        jcenter()
        google()
        flatDir {
            dirs ‘src/main/libs‘
        }
    }
    dependencies {
        classpath ‘com.android.tools.build:gradle:3.1.4‘

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
        google()
        flatDir {
            dirs ‘src/main/libs‘
        }
        maven {
            //this
            //url "https://oss.sonatype.org/content/repositories/snapshots"
            url "https://maven.google.com"
        }
    }
}