使用ES5严格模式的谷歌闭包，即使我指定了非严格模式(在minify-maven-plugin配置中)

Question

我正在使用com.samaxes.maven minify-maven-plugin来缩减使用Google Closure支持的一些ES6特性编写的JS源文件的集合。下面是我的POM中的相关配置：

<!-- minify-maven-plugin: Minification using Google Closure -->
<plugin>
    <groupId>com.samaxes.maven</groupId>
    <artifactId>minify-maven-plugin</artifactId>
    <version>1.7.6</version>
    <executions>
        <!-- Creation of the common-[version].js file -->
        <execution>
            <id>common-minify</id>
            <phase>prepare-package</phase>
            <configuration>
                <charset>UTF-8</charset>
                <jsSourceDir>.</jsSourceDir>
                <jsSourceFiles>
                    ...
                </jsSourceFiles>
                <jsFinalFile>./js/common-${project.version}.js</jsFinalFile>
                <jsEngine>CLOSURE</jsEngine>
                <closureLanguageIn>ECMASCRIPT6</closureLanguageIn>
                <closureLanguageOut>ECMASCRIPT5</closureLanguageOut>
            </configuration>
            <goals>
                <goal>minify</goal>
            </goals>
        </execution>

        <!-- 2 other similarly configured executions are here. -->
        ...

    </executions>
</plugin>

问题是，当我运行maven goal这个配置时，我得到了以下错误消息：

[INFO] Creating the merged file [common-1.8.24.js].
[INFO] Creating the minified file [common-1.8.24.min.js].
Jan 03, 2017 12:03:06 PM com.google.javascript.jscomp.LoggerErrorManager println
SEVERE: [1mcommon-1.8.24.js:5577: [31mERROR[39m - object literals cannot contain duplicate keys in ES5 strict mode[0m
    supportsDataForwarding: function () {
    ^ 

在我看来，即使我在我的<closureLanguageOut>选项(see doc here)中指定了非严格ECMASCRIPT5模式，谷歌闭包仍试图使用ES5严格模式进行编译。为什么不禁用严格模式？