Dart: web_gl: getAttributeLocation returns -1

Question

Dart: web_gl: getAttributeLocation returns -1

A call getAttributeLocation(program,"aTextureCoords")in my code returns -1

However:

The attribute is active (i.e. the question getAttribLocation Webgl is not duplicated, strange returns -1 )
Correctly written
It has no invalid characters
It does not start with a reserved prefix.
It is shorter than 255 characters
Missed program is not invalid

What could be wrong?

My (built-in) shader code:

<script type="x-shader/x-vertex" id="vertex">
    attribute vec3 aVertexPosition;
    attribute vec2 aTextureCoords;

    uniform mat4 uMVMatrix;
    uniform mat4 uPMatrix;

    varying highp vec2 vTextureCoords;

    void main() {
        gl_Position = uPMatrix*uMVMatrix*vec4(aVertexPosition,1.0);
        vTextureCoords = aTextureCoords;
    }
</script>
<script type="x-shader/x-fragment" id="fragment">
    varying highp vec2 vTextureCoords;

    uniform sampler2D uSampler;

    void main() {
        gl_FragColor = vec4(1.0,0.0,0.0,1.0);
        texture2D(uSampler,vec2(vTextureCoords.s,vTextureCoords.t));
    }
</script>