Firebase-perf conflicting with let plugin

We were recently asked to implement "Firebase Performance Monitoring" on an Android app, but it's been causing us lots of different problems. The app used to work just fine, but upon adding "firebase-perf", it compiles, but then on runtime we found out the Let plugin stopped working, every permission defined with the @AskPermission annotation is simply ignored.

Looking through the gradle build output, I stumbled upon this stack-trace:

java.lang.IllegalStateException: Expecting .,<, or ;, but found firebaseperf while unpacking <K:Ljava/lang/Object;>Lcom/google/android/gms/internal/firebase-perf/zzw<TK;>;
    at org.aspectj.util.GenericSignatureParser.parseClassTypeSignature(GenericSignatureParser.java:204)
    at org.aspectj.util.GenericSignatureParser.parseAsClassSignature(GenericSignatureParser.java:56)
    at org.aspectj.weaver.UnresolvedType.forGenericTypeSignature(UnresolvedType.java:274)
    at org.aspectj.weaver.bcel.BcelWorld.addSourceObjectType(BcelWorld.java:482)
    at org.aspectj.weaver.bcel.BcelWorld.addSourceObjectType(BcelWorld.java:456)
    at org.aspectj.weaver.bcel.BcelWeaver.addAspectsFromJarFile(BcelWeaver.java:263)
    at org.aspectj.weaver.bcel.BcelWeaver.addLibraryJarFile(BcelWeaver.java:236)
    at org.aspectj.ajdt.internal.core.builder.AjBuildManager.initBcelWorld(AjBuildManager.java:874)
    at org.aspectj.ajdt.internal.core.builder.AjBuildManager.performBuild(AjBuildManager.java:249)
    at org.aspectj.ajdt.internal.core.builder.AjBuildManager.batchBuild(AjBuildManager.java:185)
    at org.aspectj.ajdt.ajc.AjdtCommand.doCommand(AjdtCommand.java:112)
    at org.aspectj.ajdt.ajc.AjdtCommand.runCommand(AjdtCommand.java:60)
    at org.aspectj.tools.ajc.Main.run(Main.java:371)
    at org.aspectj.tools.ajc.Main$run.call(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:133)
    at com.canelmas.let.LetPlugin$_apply_closure2_closure3.doCall(LetPlugin.groovy:66)
    ...

I've seen some similar cases here and there but couldn't find a solution yet. The way I see it, it looks like a conflict between differing versions of aspectjs, defined in different classpaths.

This is the relevant part of out project-level gradle file:

buildscript {
    repositories {
        google()
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
        classpath group: 'org.tmatesoft.svnkit', name: 'svnkit', version: '1.7.11'
        classpath('com.canelmas.let:let-plugin:0.1.11')
        classpath 'com.google.gms:google-services:4.2.0'
        classpath 'com.google.firebase:firebase-plugins:1.1.5'
    }
}

And these are the firebase dependencies we have defined in the app-level gradle:

dependencies {
    compile 'com.google.firebase:firebase-core:16.0.7'
    compile 'com.google.firebase:firebase-perf:16.2.3'
}

Does anyone have an idea of a possible fix?

EDIT: There was someone in another thread, who claimed to have solved the problem with the code below, but I have no idea how to actually use it. Could anyone more Gradle-savvy be able to explain where is this declaration supposed to go and what I'm supposed to do with it?

I tried placing it many different places of my gradle file but I'm getting this error: "No such property: classpath for class: org.gradle.api.tasks.compile.JavaCompile"

def filtered_class_filetree = javaCompile.classpath.asFileTree.filter {
    !it.canonicalPath.contains("firebase-perf")
}

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