If you are recently update on your android studio to be a uptodate version than may be you can face
this common problem on your previews android studio project. The project compiled before without any changes in that file, you are getting this error

Error:(26, 0) Gradle DSL method not found: 'runProguard()'

Solution

Instead of using runProguard in your gradle file, try using minifyEnabled.
Note: To use minifyEnabled, gradle should be updated to version 2.2 or above.
Open build.gradle on your root project file .
Old Line runProguard false make it change to

buildTypes {
    release {
        runProguard false // this line has to be changed
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}

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New line minifyEnabled false Add this line

buildTypes {
    release {
        minifyEnabled false // add this 
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}

Migrating Gradle Projects to version 1.0.0. everything is described here:
http://tools.android.com/tech-docs/new-build-system/migrating-to-1-0-0

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New Release Note

Fully Understand New Build System Please Read the Official blog New build system inside ADT and Ant
Handle local jars in tests of multidex-enabled library projects
Fix path normalization (for unzipped aars) when path is only 1 char long
Fix Jack/Jill memory setting through dexOptions.javaMaxHeapSize